Method and device for treating scoliosis

ABSTRACT

This invention relates to a spinal facet cap for treating scoliosis, the facet cap comprising a shim portion for inserting into a facet joint of a spine, and an alignment portion for maintaining alignment of the shim portion within the facet joint. The invention also provides a method for treating scoliosis, comprising implanting at least one spinal facet cap into at least one facet joint of a subject in need thereof.

RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 60/268,860, filed on Feb. 16, 2001,the disclosure of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

Scoliosis is an orthopaedic condition characterized by abnormalcurvature of the spine, with varying degrees of lateral curvature,lordosis and rotation. Despite extensive research, the pathogenesis ofscoliosis remains obscure in the majority of cases.

The vertebral column is composed of vertebra, discs, ligaments andmuscles. Its function is to provide both mobility and stability of thetorso. Mobility includes rotation, lateral bending, extension andflexion. Scoliotic curvature is associated with pathologic changes inthe vertebra and related structures. Vertebral bodies becomewedge-shaped, pedicles and laminas become shorter and thinner on theconcave aspect of the curve. Apart from the obvious physical deformity,cardiopulmonary problems may also present. As curvature increases,rotation also progresses causing narrowing of the chest cavity. Insevere deformities, premature death is usually caused by respiratorydisease and superimposed pneumonia.

Treatment options have varied little over the past few decades, and onlytwo treatments effectively help correct scoliosis: spinal bracing withexercises and surgery. A properly constructed Milwaukee or low-profilebrace will aid some patients with minor scoliosis. However, if thescoliosis progresses despite such bracing, or if there is substantialdiscomfort, surgical correction involving fusion of vertebra may berequired. Surgery has traditionally involved procedures such as theHarrington, Dwyer and Zielke, and Luque procedures which rely onimplanted rods, laminar/pedicle hooks, and screws to maintain thecorrection until stabilized by fusion of vertebrae.

Thus the goal of current surgery is to strip the paraspinal muscles fromthe lamina of vertebra to be fused, and effect correction and spinalfusion in one step. The general technique is as follows:

-   -   1. The outer cortex of the lamina and spinous processes is        removed so that raw cancellous bone is exposed.    -   2. Posterior facet joints are destroyed and usually autogenous        bone graft added. Graft is usually placed along the entire        fusion area. The fusion extends from one vertebra above the        superior end-vertebra involved in the curvature to two below the        inferior end-vertebra of the curve.    -   3. Spinal instrumentation is applied. A distraction rod allows        the spine to be ‘jacked’ up on the concave side of the curve. A        compression assembly may be used on the convex side of the curve        to ‘pull’ the curve straight. Anchors, laminar hooks, and/or        wires are placed around the lamina to provide fixation for the        rods.

Yet other surgical procedures involve memory metal implants (Sanders, AMemory Metal Based Scoliosis Correction System, CIP-Data KoninklijkeBibliotheek, Den Haag, 1993), fusion of vertebra anteriorly, usinganterior cages (e.g., Harms cage, from DePuy-AcroMed Inc.).Nevertheless, it is clear that available procedures have drawbacksincluding the requirement for substantial prosthetic implants (seeMohaideen et al., Pediatr. Radiol. 30:110-118 (2000) for a review) andcomplicated surgical procedures, often only partly correct scolioticdeformities, and result in reduced flexibility of the spine.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided aspinal facet cap for treating scoliosis, comprising a shim portion forinserting into a facet joint of a spine, and an alignment portion formaintaining alignment of said shim portion within said facet joint. Theshim portion of the spinal facet cap comprises two opposed surfaces forengaging articular surfaces of the facet joint. In one embodiment, theshim portion is of substantially uniform thickness. In anotherembodiment, the shim portion is wedge-shaped.

In one embodiment of the spinal facet cap, the alignment portioncomprises a tongue. In some embodiments, the tongue is provided with anorifice.

In another embodiment, the alignment portion comprises a boss along atleast one edge of the shim portion.

In yet another embodiment, the alignment portion comprises at least onefacet hook disposed along at least one edge of the shim portion, forreceiving either one of the superior facet or the inferior facet of avertebra. In a further embodiment, the alignment portion comprises twofacet hooks disposed along two edges of the shim portion, one said facethook for receiving the superior facet of a first vertebra, a second saidfacet hook for receiving the inferior facet of a second vertebra. Infurther embodiments in which facet hooks are provided, the alignmentportion further comprises a tongue having an orifice. In yet furtherembodiments, the thickness of the wedge-shaped shim portion, and/or thedistance between alignment portions disposed on either side of the shimportion, is adjustable.

According to another aspect of the present invention there is provided amethod for treating scoliosis in a subject in need thereof comprisingimplanting in a facet joint of the subject a spinal facet cap, saidspinal facet cap comprising a shim portion and an alignment portion formaintaining alignment of said facet cap within said facet joint, suchthat scoliosis in the subject is treated. In some embodiments, a spinalfacet cap is implanted in each of two or more facet joints of thesubject, such that scoliosis in the subject is treated.

In some embodiments of the method of the invention, the alignmentportion comprises at least one facet hook disposed along at least oneedge of the shim portion, for receiving either one of the superior facetor the inferior facet of a vertebra.

In other embodiments, the method further comprises evaluating thesubject for the number, size, shape, location, and placement of spinalfacet caps required to treat scoliosis in the subject. In yet otherembodiments, an imaging system is used to so evaluate the subject. Insome embodiments, the imaging system is a computed tomography (CT)system.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed incolour. Copies of this patent with colour drawing(s) will be provided bythe Patent and Trademark Office upon request and payment of thenecessary fee.

The invention will be described, by way of example, with reference tothe accompanying drawings, wherein:

FIG. 1 shows a posterior view of a typical scoliotic spine;

FIG. 2 shows a posterior view of a scoliotic spine corrected with spinalfacet caps according to the invention;

FIGS. 3A to 3F show further embodiments of a spinal facet cap accordingto the invention;

FIGS. 4A to 4C show a further embodiment of a spinal facet cap accordingto the invention;

FIG. 5 shows a further embodiment of a spinal facet cap according to theinvention;

FIG. 6A shows a posterior view of a scoliotic spine;

FIG. 6B shows a posterior view of the scoliotic spine of FIG. 5Acorrected with spinal facet caps according to the invention; and

FIG. 7A shows a side view of a scoliotic spine; and

FIG. 7B shows a side view of the scoliotic spine of FIG. 7A correctedwith spinal facet caps according to the invention;

FIG. 8A is a radiograph of a prosthetic model of a spine with scoliosisat the mid-lumbar level;

FIG. 8B is a radiograph of the model of FIG. 8A, with the scoliosiscorrected using spinal facet caps of the invention;

FIGS. 9A and 9B show embodiments of a facet caps according to theinvention; and

FIGS. 10A and 10B are photographs showing the facet caps of FIGS. 9A and9B, respectively, inserted into facet joints of a patient.

DETAILED DESCRIPTION OF THE INVENTION

Surgical procedures for correction of scoliosis has changed little inthe last forty years. Current procedures still rely on surgicalprincipals established in the 1960's, and involve fusion of one or morevertebrae.

Selection for the level of fusion requires analysis about a plum linefrom the sacral spinous process superior. Vertebrae that pass outsidethis line identify the curve. The most superior of the inferiorvertebrae, which bisect this line, is the inferior extent for fusion andthe most inferior of the superior vertebrae that bisect this line is thesuperior extent of fusion. All levels in-between these two identifiedlevels require fusion.

After the curve is identified, all posterior ligaments and facet jointsare destroyed by decortication and cartilage is removed at each level offusion. Instrumentation for correction of the scoliosis is placed insuch a manner to apply a distractive force on the concavity of the curveand compression on the convexity of the curve. All current systems relyon some vertebral fixation to a rod. Compressive and distractive forcesare then applied along the rod at the points of vertebrae to rodfixation.

Three forms of fixation of the posterior spine are currently available:pedicle/laminar hooks, wires and screws. Laminar hooks are placed aroundvertebrae lamina. Distractive hooks are placed pointing away from theapex of the curve and compressive hooks placed facing towards the apexof the curve. Pedicle screws are placed posterior to anterior in thevertebrae and can either have a distractive or compressive force appliedthrough the rod. Laminar wires wrap around the lamina and connect to arod at each level. The corrective force is applied as the wire tightensaround a rigid rod. The lamina and thus the vertebrae are dragged to therod.

The spinal facet cap of the invention differs from the above-describedstandard instrumentation in a number of ways. For example, as discussedabove, during standard procedures for correcting scoliosis, facet jointsare generally destroyed. In contrast, the spinal facet caps of theinvention require that the facet joints are substantially or entirelyintact. This procedure is thus expected to preserve mobility of thefacet joint. Further, rather than applying a distractive or compressiveforce through a rod, the facet cap effectively reshapes the facet joint.Such reshaping affords symmetry between left and right facet jointswhich corrects abnormal curvature. Thus, no fixation of the vertebrae isrequired; rather, there is modulation of the vertebral (facet) shape.This reshaping is expected to allow for the omission of fusing theintervening vertebral levels and multiple levels of vertebrae, which isthe result of rod fixation, thus preserving flexibility of the spine.Also, the use of conventional instrumentation with a rod concentratesthe load (i.e., weight of the torso) on the portion of the spine towhich the rod is attached, as well as on the rod itself. The resultingstress sometimes results in failure of the conventional instrumentation.In contrast, in providing for the correction of individual facet jointsof the spine, the facet cap of the invention maintains the natural loaddistribution along the spine. Accordingly, there is no loadconcentration at any point of the spine, and low likelihood of failureof the implanted facet cap.

As used herein, the term “scoliosis” is intended to mean any abnormalcurvature of the spine. Such abnormal curvature can exist in any one ofall three planes, or in any combination thereof, and hence can bemanifested by inappropriate lateral curvature, lordosis, kyphosis,and/or rotation. Scoliosis can be congenital or idiopathic, or inducedby injury, trauma, infection, inflammation, or degenerative changes inthe spine.

As used herein, the term “treating scoliosis” is intended to meancorrecting or reducing curvature of the spine of a subject, such thatthe subject experiences an improvement in condition, comfort (e.g.,reduction or amelioration of pain), appearance, posture, and/orflexibility of the spine. The term “treating scoliosis” is also intendedto mean preventing scoliosis from progressing to a more severe state, orinhibiting the degree to which scoliosis progresses.

As used herein, the term “subject” is intended to mean any vertebratethat can have scoliosis. Typically, such subjects are primates.Preferably, the subject is human.

According to one theory, many forms of scoliosis result from asymmetrybetween left and right facet joints of vertebrae. According to anothertheory, many forms of scoliosis cause asymmetry between left and rightfacet joints of vertebrae. While not holding to one particular theory atthe exclusion of others, the present invention provides for thetreatment of scoliosis by substantially or completely correcting suchasymmetry.

In one aspect, the invention provides a prosthetic device for treatingscoliosis by substantially or completely correcting asymmetry betweenleft and right facet joints of vertebrae. The prosthetic device,generally referred to as a spinal facet cap, is surgically implantedinto a spinal facet joint at any level in the spine. Surgicallyimplanting one or more spinal facet cap(s) is carried out with minimalor no modification of the facet joint(s) involved; thus, the inventionprovides for the correction of left-right asymmetry of facet jointswhile preserving the facet joints. In this respect the invention isunlike any known procedures for treating scoliosis.

FIG. 1 shows a posterior view of a typical scoliotic spine, withasymmetry between left and right facet joints, and spinal curvatureconvex left. FIG. 2 shows the spine of FIG. 1 in which the asymmetrybetween left and right facet joints has been corrected with two spinalfacet caps according to an embodiment of the present invention. As canbe seen in FIG. 2, a spinal facet cap 10 according to the inventioncomprises a shim portion 12 which is implanted between the superiorfacet 22 of a first (lower) vertebra 20 and the corresponding inferiorfacet 32 of a second overlying vertebra 30. The shim portion has twoopposed surfaces, a first (lower) surface 14 engaging the superiorarticular surface of the superior facet 22, and a second (upper) surface16 engaging the inferior articular surface of the corresponding inferiorfacet 32. The opposed surfaces of the shim portion of the spinal facetcap can be substantially planar, as shown in FIG. 2, or they can beformed (e.g., concave or convex) to receive and at least partiallycomplement or parallel superior and inferior facet contours.

From FIG. 2 it will be appreciated that the shim portion of the spinalfacet cap must be properly aligned or positioned in the facet joint, andthat this alignment must be maintained. An alignment portion is providedfor this purpose. The alignment portion can be provided numerous ways inaccordance with the invention. For example, the alignment portion cancomprise an extension or tongue 18, having an orifice 19, to accept ascrew or the like which is driven into the cortex of the vertebralpedicle. The alignment portion can also comprise one or more facet hooksand/or a ridge or boss disposed along the perimeter or margin of theshim portion, to engage the superior and/or inferior facets. Thealignment portion at least partially encompasses the superior and/orinferior facet(s).

In some embodiments, the opposed surfaces are parallel (i.e., coplanar),such that the shim portion is of substantially uniform thickness. Inother embodiments, the opposed surfaces are not coplanar, such that theshim portion is not of uniform thickness and is generally wedge-shaped.In embodiments where the opposed surfaces are not coplanar, the surfacescan be sloped along a common axis so as to form a simple angle. Theangle separating the opposed surfaces can be, for example, from 0°(coplanar) to about 40°, preferably about 0° to about 20°. In otherembodiments, the slopes of the opposed surfaces form a compound angle inwhich the slopes are not aligned on a common axis. It will beappreciated that the direction of slope is appropriately chosen tocorrect a facet joint for a given abnormality of curvature (e.g.,kyphosis, lordosis, etc.).

It will be appreciated that a spinal facet cap according to theinvention can be produced in a range of sizes and shapes by varying thethickness of the shim portion (i.e., distance between the opposedsurfaces), the angle of the shim portion (i.e., angle between theopposed surfaces), the area of the shim portion (i.e., surface area ofthe opposed surfaces), and the shape of the shim portion and/or opposedsurfaces so as to provide any desired amount of correction to a facetjoint, and to accommodate any size of facet joint. Further, a facet capaccording to the invention can be symmetrical or asymmetrical withrespect to the angle, area, and shape of opposed surfaces.

Each opposed face of the shim portion is sized to accommodate thegenerally circular or oblong (i.e., oval) shape of the articulatingsurface of a facet joint into which the facet cap is inserted. The sizeor area of the opposed faces of a facet cap will depend on factors suchas where in the spine the facet cap is implanted (e.g., cervical beingsmaller than lumbar), the age and size of the subject, and the conditionof the facet joint. The size of the opposed faces is generally describedherein as circular, having a diameter; however, it is to be understoodthat the opposed faces can be of any shape and the diameter refers tothat portion of an opposed surface that substantially corresponds to anarticular surface of a facet joint. The diameter of each opposed face ofthe shim portion ranges from about 5 mm to about 30 mm, preferably fromabout 10 mm to about 20 mm, still more preferably from about 12 mm toabout 18 mm.

It will be appreciated that patients receiving facet caps can vary inage/size, and in degree of severity of scoliosis. Thus, the presentinvention contemplates a range of sizes and shapes of facet caps, tosuit any facet joint in need of treatment, so as to correct any or allof a reduction in height, an abnormality in tilt, and an abnormality inangulation (e.g., kyphosis, lordosis) of the inferior or superiorvertebral body. The facet caps can be provided ready for implanting(e.g., sterilized and appropriately packaged), or they can be sterilizedprior to implanting using methods well-known in the art.

A spinal facet cap according to the invention can be made of anysuitable biologically inert material. Examples of suitable materials arecobalt chrome, stainless steel, and titanium. Cobalt chrome is preferredowing to its biocompatibility with tissues and cartilage.

Several embodiments of the spinal facet cap of the present invention areshown in FIG. 3. For example, FIG. 3A shows a spinal facet cap like thatshown in FIG. 2. In FIG. 3B, the shim portion 50 has an alignmentportion comprising a ridge 54 disposed along the edge of and partiallysurrounding the surface 56 that engages the inferior facet, toward thetongue 52. As shown in FIGS. 3C and 3D, in which FIG. 3D shows alongitudinal section of the embodiment of FIG. 3C, the shim portion 60similarly has an alignment portion comprising a ridge 68 on the edge ofthe surface 67 that engages the superior facet, opposite the tongue 62.The embodiment of FIGS. 3C and 3D also has a further ridge 64 on theedge of the surface 66 that engages the inferior facet, toward thetongue 62. It will be appreciated that the provision of an alignmentportion comprising ridges on the first, second, or both surfaces of thefacet cap helps to maintain alignment of the facet cap with the superiorand inferior facets, and helps to keep the facet cap registered in thefacet joint.

The alignment portion of a spinal facet cap according to the inventioncan also comprise one or more pins extending outwardly from at least oneof the two opposed surfaces. For example, the spinal facet cap 70 shownin FIG. 3E has pins 72, 74 extending outwardly from the opposed surfaces73, 75, respectively. The pins 72, 74 engage holes prepared in thearticular surfaces of the superior and inferior facets during theimplant procedure.

In FIG. 3F there is shown another embodiment of a spinal facet capaccording to the invention in which the shim portion 80 has an alignmentportion comprising a facet hook 88 on the edge of the surface 87 thatengages the superior facet, substantially opposite the tongue 82, andanother facet hook 84 on the edge of the surface 86 that engages theinferior facet, toward the tongue 82. In further embodiments, only oneof either facet hook 84 or facet hook 88 is present. The facet hook canvary in the extent of the curvature of the hook and thus the extent towhich the hook encompasses the inferior/superior facet. For example, insome embodiments the curvature of the facet hook can be reduced so thatthe hook extends from the shim portion in a 90° arc, whereas in otherembodiments the hook extends from the shim portion in a 180° arc.

Although not shown in the drawings, it will be appreciated thatembodiments of the invention such as those shown in FIGS. 3A to 3D and3F can be provided with an orifice passing through the shim portion, foraccepting a pin, screw, or the like driven through at least one of theinferior and superior facets, to thereby contribute to maintainingalignment of the spinal facet cap. In particular, in the embodiment ofFIG. 3F, each of the facet hook 84, shim portion 80, and facet hook 88can be provided with an orifice, the three orifices having a commonlongitudinal axis, so as to accommodate a pin or screw disposed throughthe facet hook 84, the inferior facet, the shim portion 80, the superiorfacet, and the facet hook 88. Also not shown in the figures areembodiments in which the shin, portion is sloped in the oppositedirection to that shown (i.e., a directive relative to the tongue 18 inFIG. 3A).

In the embodiment shown in FIGS. 4A to 4C, which is similar to that ofFIG. 3F, the spinal facet cap 90 has a shim portion 92 with opposedsurfaces 94, 96 provided with an alignment portion comprising facethooks 95, 97, respectively, and a tongue 98. Tongue 98 extends outwardlyfrom the shim portion 92, and has an orifice 99 for accepting a screw.Facet hook 95 engages the inferior facet, and facet hook 97 engages thesuperior facet. This can be seen in FIG. 6, which shows a scolioticspine (FIG. 6A) in which the decrease in height and lateral tilt of thespine have been corrected with spinal facet caps according to thepresent embodiment (FIG. 6B). A handle 100 is optionally provided tofacilitate implanting the facet cap. The handle 100 is attached to thefacet cap in a manner to allow it to be removed upon implanting the cap.For example, the handle 100 can be crimped at the junction with thefacet cap, so that it can simply be broken off once the facet cap isimplanted.

It will be appreciated that the embodiment shown in FIG. 4 can beprovided with only a single facet hook, in which case it is preferablethat the facet hook 95 that engages the inferior facet is provided.However, the provision of two facet hooks 95 and 97 improves thestability of the implant. Also, the facet hooks can be wider or narrowerthan those shown in FIG. 4. A wider facet hook has the advantage ofcontacting more of the facet, and hence is preferable. When a very widefacet hook is provided, it can be curved so as to approximate the shapeof the portion of the facet that it contacts, and hence contact agreater portion of the facet. As an alternative to a wide facet hook, afacet hook can comprise two or more fingers, the fingers providingmultiple points of contact with a facet. An advantage of such fingers isthat growth of tissue around and between the fingers is possible, andsuch growth improves the stability and reliability of the implant.

In a further embodiment, the distance between facet hooks, and/or theangle of the shim portion (i.e., the extent to which the shim portion iswedge-shaped) can be adjusted. An example of this embodiment is shown inFIG. 5. FIG. 5A shows this embodiment, denoted by reference numeral 120,in side and plan views, which comprises two parts 130 and 150. FIG. 5Bshows part 130 in side and plan views, and FIG. 5C shows part 150 inside and plan views. Part 130 comprises a plate 134, an inferior facethook 132 disposed on a first surface of the plate 134, and a tongue 140and two rows of teeth 138 disposed on the opposite surface of the plate134. An orifice 136 accommodates a cortical screw (not shown). Part 150comprises a plate 154, a superior facet hook 152 disposed on a firstsurface of the plate 154, and two rows of teeth 158 disposed on theopposite surface of the plate 154. A longitudinal opening 156 isprovided in the plate 154, for accepting the tongue 140 of part 130 in asliding fit. As can be seen from FIG. 5A, part 130 mates with part 150such that tongue 140 fits in opening 156 and teeth 138 mesh with teeth158, and the inferior and superior facet hooks 132 and 152 are opposed.When mated, plates 134 and 154 comprise the shim portion. Preferably,the teeth comprising each set of teeth 138 and 158 are asymmetrical,such that meshing of the two sets of teeth forms a ratchet that allowsparts 130 and 150 to slide relative to each other in one direction, butnot the other. Preferably, such ratchet allows parts 130 and 150 toslide in a direction which brings the inferior and superior facet hookscloser together, and prevents the facet hooks from sliding furtherapart. Thus, to use this embodiment to correct a facet joint, parts 130and 150 are first mated such that the facet hooks are farthest apart,and the so-assembled facet cap is inserted into a facet joint of apatient. The distance between the facet hooks is then reduced by slidingparts 130 and 150 together, to fit the facet joint being corrected andto provide the desired amount of correction. In variations of thisembodiment, either or both of plates 134 and 154 can be wedge-shaped, sothat as parts 130 and 150 slide relative to each other, the amount ofshim provided to a facet joint can be adjusted. Further, suchwedge-shape of part 130 and/or part 150 can be tapered in any directionrelative to the facet hook, so as to provide correction for any type offacet joint asymmetry (e.g., lordosis, kyphosis, etc.). It will beappreciated that the facet hooks in this embodiment could be substitutedfor ridges, bosses, etc, as discussed in respect of the alignmentportion of the above embodiments.

FIG. 7A shows a side view of a scoliotic spine with pronounced lordosis.In FIG. 7B, the scoliosis shown in FIG. 7A has been corrected byinserting spinal facet caps 90 between facet joints. Here, spinal facetcaps according to the embodiment shown in FIG. 3F or FIG. 4 areemployed.

In another aspect, the invention provides a method for treatingscoliosis by surgically implanting at least one spinal facet cap into atleast one spinal facet joint, such that asymmetry between left and rightfacet joints of vertebrae is substantially or completely corrected.

Treating a subject exhibiting mild scoliosis might involve implantingonly a single facet cap. Treating more severe scoliosis might involveimplanting two or more spinal facet caps at various locations in thespine to achieve the desired correction. In such severe cases, spinalfacet caps of various sizes and shapes are expected to be employed. Asmentioned above, the invention provides for treating scoliosis whilepreserving facet joints. However, in some cases, some minor modification(e.g., providing a hole for a pin) or more extensive modification (e.g.,removal of bone) of either or both of the inferior and superior facetsmight be necessary or desirable to ensure proper seating and alignmentof a spinal facet cap, and hence improve the subject's prognosis.Treating a subject with scoliosis involves evaluating the subject priorto surgery and during surgery for the number, size, shape, location, andplacement of spinal facet caps to achieve the desired correction. Animaging system (e.g., computed tomography (CT), radiography, or magneticresonance imaging (MRI)) can advantageously be used to evaluate thesubject prior to surgery, to help determine the number, size, shape,location, and placement of spinal facet caps to achieve the desiredcorrection. Further, data obtained from such evaluation of a subjectprior to surgery can be used to prepare a custom suite of facet caps of,for example, various sizes and shapes, to suit a particular subject.

WORKING EXAMPLES Example 1 Cadaveric Implantation

A spinal facet cap based on the embodiment shown in FIGS. 2 and 3A,having a diameter of about 12 mm, was surgically implanted into thescoliotic spine of a cadaver (female, elderly) at the Department ofAnatomy and Cell Biology at Queen's University, Kingston, Ontario,Canada, to evaluate the ease or difficulty of placement and the seatingof the facet cap in the facet joint. There were no complications inimplanting the facet cap into the spine, suggesting that use of thefacet cap for treating scoliosis could become a routine surgicalprocedure. Moreover, during this exercise it was found that seating ofthe facet cap in the facet joint was fully satisfactory. This exercisetherefore provides a strong indication that the spinal facet cap of theinvention will be effective in the treatment of scoliosis.

Example 2 Prosthetic Model

Osteotomies were performed on the mid-lumbar facets of a prostheticmodel of an adult human spine to create a scoliotic model. This is shownin the radiograph of FIG. 8A, where reference numeral 200 refers to pinsused to hold the model together. Spinal facet caps like that shown inFIG. 3F were then inserted into the mid-lumbar facet joints, whichsubstantially corrected the scoliotic curvature of the spine. This canbe seen in the radiograph of FIG. 8B, where reference numeral 210 refersto the facet caps.

Example 3 Formulation of In Vivo Placement

The success of cadaveric implantation led to the formulation of tempo invivo application. During the course of standard scoliosis surgery, allfacet joints from the superior to inferior aspect of the proposed fusionlevels are stripped of the joint capsule, the cartilage removed, and thejoint decorticated. Prior to destruction of the joints an in vivo modelfor facet cap placement is present, as no further dissection of thespine is necessary, in which the facet caps can be placed and removed inminutes. This has allowed the formulation of a working model forapplication of the facet caps. Thus, the below examples relate to thetemporary insertion of facet caps during the course of standardcorrective surgery, to establish an operative technique for theirinsertion, and to evaluate their efficacy and ease of use.

Example 4 Operative Technique Pre-operative

Pre-operative planning is based on the standard standing radiograph ofthe spine. The most accessible inferior vertebral body demonstratingtilting relative to pelvis is identified. The inferior tilting of thevertebra is measured to determine the appropriate thickness of the shimportion and distance between facet hooks of the facet cap. The nextsuperior adjacent facet may also be targeted as a sight for correction.More superiously in the spine the apex of the scoliotic curve isidentified. A measurement of the interior tilt of this vertebra isobtained along the concave aspect of the scoliotic curvature. A facetcap having a shim of appropriate thickness and distance between facethooks is placed at this level and the next superior adjacent facet mayalso be targeted.

Operative Technique

The patient is placed prone, supported by bolsters over ASIS and upperchest with care to keep pressure off the abdomen. After preparing theskin the back is draped to expose the midline of the back.

A midline incision is made over the spinous processes over theappropriate levels (see pre-operative planning). The linea between theleft and right paravertebral muscles is dissected down to the spinousprocesses. Localization of the vertebral levels is checked by APradiograph. The paravertebral muscles are then reflected laterally alongthe lamina to the facet joints. Care is taken to maintain the integrityof the facet ligaments. Further soft tissue dissection is then performedto expose the transverse processes.

The facet joint, which is to receive the facet cap, is then stripped ofthe joint capsule and posterior pericapsular ligament. Care is taken toleave the facet cartilage intact. The contra lateral facet joint is thenstripped of ligament and capsule. The facet joint cartilage on thecontra lateral facet joint is excised to bleeding subchondral bone. Alaminar spreader is placed between the superior and inferior transverseprocesses on the side of the spine to receive the facet cap. The laminarspreader is distracted to open the space in the ipslateral facet joint.The facet cap is then placed in the facet joint and laminar spreaderremoved. Correct placement of the facet cap should allow for maintenanceof the distraction created by the laminar spreader. The bone is thendecorticated, autologus bone graft placed along both sides of thetransverse processes, facet and lamina. AP and lateral radiographs aretaken to assess position and affect of the facet cap. The paravertebralmuscle is then approximated and skin closed.

Example 5

Male patient 16 years old with 70 degree thoracic curve and 95 degreeneuromuscular kypho-scoliosis (Kingston, Ontario, Canada). Thepre-operative plan for facet cap placement was to assess the possibilityof seating a facet cap like that of FIG. 3F in the lumbar facet jointbetween the L1 and L2 lumbar vertebrae.

The posterior spine was prepared in the standard fashion, describedabove. After the L1-L2 lumbar vertebrae facet was stripped of thecapsule the joint was inspected. No space was available to open thefacet joint so that the facet cap could be inserted. However, it isexpected that could the facet joint have been opened, the facet positioncould be altered to allow the facet cap to be seated.

Example 6

Female patient 14 years old with 54 degree King II idiopathic scoliosis(Kingston, Ontario, Canada). The pre-operative plan was for facet capplacement in the L1-L2 lumbar facet joint, and the T6-T7 thoracic facetjoint. The use of laminar spreader distraction between the facet jointaided in facet joint alignment, as did the addition of a small metaldissector into the joint for space creation and facet cap placement.

The posterior spine was prepared in the standard fashion as describedabove. After the L1-L2 lumbar vertebrae facet was stripped of thecapsule, a blunt osteotome was placed in the inferior joint and wedgedthe joint open. Concurrently a laminar spreader was placed between L1and L2 and distracted. These two manoeuvres opened the facet joint andsubjectively corrected the scoliosis in this segment. However, the facetcap like that of FIG. 3B could not be inserted because it was the wrongsize for this patient. In particular, the facet hook pattern (radius) ofthe superior and inferior facet hooks of the facet cap was too narrowand the overall length of the facet cap too long. It is expected thatchanging the radius of the facet hooks to a range of about 5 mm to about1 cm, and the overall length of the facet cap to a range of about 1.5 cmto about 2.5 cm would have been appropriate. The thoracic facet wasaddressed, and it was found that the superior facet hook obscuredimplantation of the facet cap in this patient and the facet cap couldnot be inserted. It is expected that rotation of the inferior facet hookby about 30 degrees from parallel to the facet cap to the right, forright insertion, and to the left for left insertion, would havefacilitated implantation of the facet cap in this patient.

Example 7

Female patient 16 years old with 45 degree thoracic curve and 95 degreekyphosis neuromuscular kypho-scoliosis (Kingston, Ontario, Canada). Thepre-operative plan was to place modified facet caps (FIGS. 9A and 9B)into the L2-L3 lumbar facet joint and T7-T8 thoracic facet joint.

The posterior spine was prepared in the standard fashion, describedabove. Using the technique described above the L2-L3 facet joint wasopened and a facet cap like that shown in FIG. 9B was inserted. This isshown in FIG. 10B, where reference numeral 400 refers to the facet cap,with superior facet hook 410 and inferior facet hook 420 partiallyvisible. Also shown in FIG. 10B are several Moss® Miami (DePuy-AcroMedInc.) laminar hooks 430 placed around vertebral laminae, for use withrods for the standard corrective procedure. The facet cap subjectivelycorrected the scoliosis at this level. The T7-T8 facet joint wasprepared and a facet cap like that shown in FIG. 9A was inserted, asshown in FIG. 10A where reference numeral 300 denotes the facet cap. Theinferior facet hook 320 can be seen clearly. Also visible are severalMoss® Miami (DePuy-AcroMed Inc.) laminar hooks 330 placed aroundvertebral laminae, for use with rods for the standard correctiveprocedure. The facet cap subjectively corrected the scoliosis at thislevel. Dimensions of the facet caps used in this example are provided inthe below table.

TABLE 1 Dimensions of facet caps used in Example 7 and shown in FIGS. 9Aand 9B. Width of both facet caps was about 12.7 mm. Dimension mm a 2.0 b1.5 c 25.0 d 28.0 e 14.7 f 8.7 g 8.4 h 0 r 3.2

EQUIVALENTS

Variants to the embodiments described above will be apparent to thoseskilled in the art. Such variants are within the scope of the presentinvention and are covered by the below claims.

1-27. (canceled)
 28. A prosthetic device, comprising: a shim portionadapted for insertion into a facet joint of the spine, the shim portionincluding substantially opposed surfaces for engaging articular surfacesof the facet joint, the opposed surfaces being at least partiallycomplementary to articular surfaces of the facet joint; and means formaintaining alignment of the shim portion within the facet joint. 29.The prosthetic device of claim 28, wherein thickness of the shim portionis adjustable.
 30. The prosthetic device of claim 28, wherein the meansfor maintaining alignment of the shim portion comprises at least onefacet hook extending from the shim portion, the at least one facet hookadapted for receiving at least a portion of either the superior facet orthe inferior facet of a vertebra.
 31. The prosthetic device of claim 30,wherein the at least one facet hook includes a curved portion forreceiving at least a portion of either the superior facet or theinferior facet of the facet joint.
 32. The prosthetic device of claim31, wherein the at least one facet hook is curved in a direction that atleast partially overlies a said opposed surface of the shim portion. 33.The prosthetic device of claim 30, wherein a relative distance betweenthe shim portion and the at least one facet hook is adjustable.
 34. Theprosthetic device of claim 30, comprising at least two facet hooks, theat least two facet hooks having curved portions for receiving at least aportion of either the superior facet of the facet joint or the inferiorfacet of the facet joint.
 35. The prosthetic device of claim 30,comprising at least two facet hooks, a first facet hook having a curvedportion for receiving at least a portion of the superior facet of thefacet joint, and a second facet hook having a curved portion forreceiving at least a portion of the inferior facet of the facet joint.36. The prosthetic device of claim 28, wherein the means for maintainingalignment of the shim portion comprises an alignment portion, thealignment portion adapted for being fixed to a vertebral structureassociated with a superior facet or an inferior facet of the facetjoint.
 37. The prosthetic device of claim 36, wherein the alignmentportion has a hole for accepting a screw, the screw fixing the alignmentportion to a vertebral structure associated with the superior facet orthe inferior facet of the facet joint.
 38. The prosthetic device ofclaim 28, wherein the means for maintaining alignment of the shimportion comprises: (i) at least one facet hook extending from the shimportion, the at least one facet hook adapted for receiving at least aportion of either the superior facet or the inferior facet of avertebra; and (ii) an alignment portion, the alignment portion adaptedfor being fixed to a vertebral structure associated with a superiorfacet or an inferior facet of the facet joint.
 39. The prosthetic deviceof claim 38, wherein the at least one facet hook includes a curvedportion for receiving at least a portion of either the superior facet orthe inferior facet of a vertebra.
 40. The prosthetic device of claim 38,wherein the alignment portion has a hole for accepting a screw, thescrew fixing the alignment portion to a vertebral structure associatedwith the superior facet or the inferior facet of the facet joint.
 41. Amethod for treating an ailment of the spine in a subject in needthereof, comprising: implanting in a facet joint of the spine aprosthetic device comprising (i) a shim portion adapted for insertinginto a facet joint of the spine, the shim portion includingsubstantially opposed surfaces for engaging articular surfaces of thefacet joint, the opposed surfaces being at least partially complementaryto articular surfaces of the facet joint, and (ii) means for maintainingalignment of the shim portion within the facet joint; wherein abnormalcurvature of the spine is treated and mobility of the spine issubstantially preserved.
 42. The method of claim 41, comprising:implanting in each of two or more facet joints of the subject aprosthetic device comprising (i) a shim portion adapted for insertinginto a facet joint of the spine, the shim portion includingsubstantially opposed surfaces for engaging articular surfaces of thefacet joint, the opposed surfaces being at least partially complementaryto articular surfaces of the facet joint, and (ii) means for maintainingalignment of the shim portion within the facet joint; wherein abnormalcurvature of the spine is treated and mobility of the spine issubstantially preserved.
 43. The method of claim 41, wherein the ailmentof the spine is associated with at least one of scoliosis, lordosis,kyphosis, and inappropriate lateral curvature in the subject.
 44. Themethod of claim 41, wherein the ailment of the spine is congenital oridiopathic or induced by one or more of injury, trauma, infection,inflammation, and degenerative changes in the spine.
 45. The method ofclaim 41, wherein implanting the prosthetic device includes implanting adevice wherein the means for maintaining alignment of the shim portioncomprises at least one facet hook extending from the shim portion, theat least one facet hook adapted for receiving at least a portion ofeither the superior facet or the inferior facet of a vertebra.
 46. Themethod of claim 41, wherein implanting the prosthetic device includesimplanting a device wherein the means for maintaining alignment of theshim portion comprises an alignment portion, the alignment portionadapted for being fixed to a vertebral structure associated with asuperior facet or an inferior facet of the facet joint.
 47. The methodof claim 41, wherein implanting the prosthetic device includesimplanting a device wherein the means for maintaining alignment of theshim portion comprises: (i) at least one facet hook extending from theshim portion, the at least one facet hook adapted for receiving at leasta portion of either the superior facet or the inferior facet of avertebra; and (ii) an alignment portion, the alignment portion adaptedfor being fixed to a vertebral structure associated with a superiorfacet or an inferior facet of the facet joint.